Roche Holding has offered to buy the rest of Genentech, about a 44% stake it doesn't already own, for $89 a share, or about $43.7 billion.

The deal would mark one of the Swiss drug maker's biggest acquisitions and would mean about $750 million to $850 million in pre-tax savings.

The offering price represents a premium of 8.8% to South San Francisco-based Genentech's (DNA) Friday's closing price.

The Basel-based drug maker said it expects to increase research productivity and cut costs by combining operations. Genentech has facilities in Vacaville, Oceanside, Hillsboro, Ore., and Singapore.

Staffing changes and other details were not disclosed.

"The transaction will also unlock synergies by leveraging the scale of the combined operations in the U.S. and improving operational efficiency," said Roche chief executive Severin Schwan.

Roche said the combined company would be the seventh largest pharmaceutical operation in the U.S. in terms of market share and would bring in more than $15 billion in annual U.S. revenue.

Genentech said it expects that a special committee of its board, composed of independent directors, "will be convened promptly to determine what action to take with respect to the proposal."

Genentech shares soared on the news Monday, hitting $92.98, up 14%, in mid-morning trading.